The invention relates to the field of circuitry for isolating digital electronic signals. Such circuits are used to provide galvanic isolation between digital signal sources in a process control system and micro-controllers receiving signals from these sources, or between micro-controllers and other digital signal sources and transducers or other devices using those signals.
In many industrial applications, such as process control systems or data acquisition and control systems, digital signals need to be transmitted between sources and user interfaces to use these signals. To maintain safety voltage levels at the user interface and prevent transient signals to be transmitted from the sources, electrical isolation is mandatory. There are three commonly known isolation methods: opto-couplers, capacitively coupled isolators, and transformer based isolators.
Opto-couplers have limitations in their lifetime, speed and power dissipation, while both capacitively coupled isolators and transformer-based isolators are essentially AC coupled, and have limitations in their size and the ability to reject common mode voltage transients. All of the three types of isolators suffer from their difficulties in IC integration, and sometimes they even require hybrid packaging.
Recently, a new isolation technique based on giant magneto-resistive (GMR) resistors has been proposed. These GMR resistors are typically magnetic multilayers that consist of thin magnetic layers separated by thin non-magnetic layers. Circuits made with this GMR based isolation technique allow multiple high voltage isolated inputs to be economically integrated with conventional VLSI circuitry. There also exist methods of using a Faraday shield to improve the ability to reject the common voltage transients in these magnetically coupled isolators.
Among various GMR structures, there is one type of structure that consists of a sandwich of a couple of ferromagnetic (FM) layers separated by a non-magnetic layer on the top of an anti-ferromagnetic (AFM) layer. The AFM layer pins the spin orientation of the neighboring FM layer and the spin direction of the other FM layer is relatively free to be switched by an applied magnetic field. When the two FM layers are ferromagnetically coupled, i.e., the magnetizations in the two FM layers are parallel, the structure is at a state of lower resistance.
On the other hand, when the two magnetic layers are anti-ferromagnetically coupled, i.e., the magnetizations in the two FM layers are anti-parallel. The structure is at a state of higher resistance. This type of structure can be switched between two magnetic states acting like a valve, and is conventionally referred to as a spin valve. Due to the spin valves' unique magnetic transport properties, it is possible to utilize them to make digital isolators that dissipate much less power compared to one based on other GMR resistors, let alone compared to one based on the three conventional types of isolation methods.